Go pally for tank... It has been talked to death in these forums that pally are the best MB tanks.

For healing i would say go with a priest. Reason is that with more then 1 lock (some could agrue that even with 1) priest are the best because of there AoE heal. Think about it, you have 3 locks and they use all there mana instead of drinking you can life tap and then heal them and move on with almost no down time. Granted a sham has chain heal but each heal is less then the first, with the priest it is the same plus you can heal the tank. I went with a priest for my 3 locks and pally team.